This invention relates to a method by which laser light is allowed to pass many times through a nonlinear optical crystal so that the efficiency of wavelength conversion of the incident laser light is sufficiently enhanced to generate higher harmonic waves (light) such as second, third and fourth harmonics (light).
Conventionally, wavelength conversion of laser light is accomplished by single pass of the laser light through a nonlinear optical crystal. A typical application of this wavelength conversion technique is the generation of a second harmonic wave, as shown schematically in FIG. 1. Incident laser light 1 is allowed to pass only once through a single nonlinear optical crystal 2, whereupon a second harmonic wave (light) 3 is obtained together with the laser light 1 that has simply passed through the crystal without experiencing a conversion in wavelength.
On account of several limitations including nonuniformity in the temporal and spatial intensity of the incident laser light and phase mismatch in the nonlinear optical crystal, the system shown in FIG. 1 allows for only a single pass of the incident laser light through the nonlinear optical crystal and the efficiency of wavelength conversion that can be achieved is usually no more than about 50%. In other words, about one half of the energy of the incident laser light is simply wasted without contributing to the conversion of its wavelength.
An object, therefore, of the present invention is to provide a method for efficient wavelength conversion of laser light by allowing it to pass a multiple of times through more than one nonlinear optical crystal.
This object of the invention can be attained by providing two nonlinear optical crystals in a wavelength conversion system. The incident laser light whose wavelength was not changed by the first nonlinear optical crystal is launched into the second nonlinear optical crystal, where it undergoes due wavelength conversion. As a result, the energy of the incident laser light is efficiently utilized in wavelength conversion.
Specifically, the two nonlinear optical crystals are disposed such that their optical axes (e-axes) are perpendicular to each other. With this arrangement, the higher harmonic wave generated by passage through the first nonlinear optical crystal does not pass along the e-axis of the second nonlinear optical crystal and, hence, will not revert to the original incident laser light upon leaving the second nonlinear crystal. As a result, the incident laser light is efficiently converted in wavelength.
More specifically, the two nonlinear optical crystals are combined with a returning mirror and a polarizer so that the rotation of polarized light is utilized to allow the incident laser light to undergo multiple passes through the two nonlinear optical crystals. Given this parts arrangement, the incident laser light that was not heretofore converted in wavelength by single pass is reflected and allowed to pass many times through the two nonlinear optical crystals. As a result, the incident laser light is efficiently converted in wavelength.